1. Field of the Invention
The present invention relates to a control system for an automatic transmission of an automobile, particularly to a control system for automatically controlling the gear position of the transmission.
2. Description of the Prior Art
In conventional automatic transmission control systems, the gear position is automatically shifted based on a predetermined shift pattern, in accordance with the automobile speed, sensed by an automobile speed sensor, and with the throttle valve position, sensed by a throttle angle sensor.
The best gear position differs depending upon the engine torque and automobile speed. Therefore, ideally, the gear position should be shifted in accordance with engine torque and automobile speed. Since is difficult to measure engine torque, conventional systems have used the throttle valve position instead. The throttle valve position, however, does not perfectly reflect the engine torque. For example, engine aging sometimes results in a difference between the throttle valve position and the engine torque. This in turn results in control of the gear position out of correspondence with the true change of the engine load, decreasing driving efficiency. When the engine is running on a very lean gasoline mixture in response to certain engine conditions, the driveability becomes particularly poor.